People who regularly smoke five joints a day had dampened reactions to the chemical messenger

People who use marijuana heavily appear to have blunted brain responses to dopamine, a chemical messenger associated with pleasant feelings, motivation and reward. This diminished response might help explain why some people abuse cannabis, scientists write July 14 in the Proceedings of the National Academy of Sciences.

Nora Volkow, director of the National Institute on Drug Abuse in Rockville, Md., and colleagues tested brain and body responses of 24 heavy marijuana users. On average, these people smoked nearly five joints a day, five days a week and had been using marijuana for a decade.

These marijuana users had weaker responses to the stimulant methylphenidate, which is used to treat attention-deficit/hyperactivity disorder, than nonusers, the team found. Users reported a less intense “high” from methylphenidate, which increases dopamine in the brain. Bodily responses were also blunted: In users, the stimulant didn’t affect pulse rate and diastolic blood pressure as strongly. The dampened dopamine response might lead to marijuana cravings, researchers write.